With the popularity and continuing development of devices such as cellular phones in recent years, higher capacities are being desired for the batteries used as power sources. Of particular note are metal-air batteries, as high-capacity batteries that have high energy density and are superior to the currently widely used lithium ion batteries, because oxygen in the air is used as the positive electrode active material to conduct oxidation-reduction reaction of the oxygen at the air electrode, while at the negative electrode, oxidation-reduction reaction of the metal composing the negative electrode is conducted, thereby allowing charge or discharge (NPL 1).
Organic solvents have conventionally been used as non-aqueous electrolytes in metal-air batteries, but the organic solvents are volatile and have problems of miscibility with water, and therefore stability for prolonged operation has been an issue. Prolonged battery operation is associated with risks, as the battery resistance increases with volatilization of the electrolyte solution from the positive electrode (air electrode) end, or penetration of moisture into the battery results in corrosion of the metal lithium serving as the negative electrode. This phenomenon can lead to loss of the prolonged discharge capacity that is a feature of air batteries.
An air battery employing an ionic liquid such as N-methyl-N-propylpiperidinium bis(trifluoromethanesulfonyl)amide (PP13TFSA) as the non-aqueous electrolyte has been proposed (PTL 1), with the aim of providing a lithium-air battery that inhibits reduction of the electrolyte solution due to volatility and inclusion of moisture into the battery, and that is capable of stable battery operation for long periods. An ionic liquid is a substance composed of ion molecules that are combinations of cations and anions, and that is liquid at ordinary temperature (15° C. to 25° C.)